The invention relates to a novel optical disk and, more particularly, to that disk, to a method of recording and reading information on the disk and to apparatus for carrying out that method.
Optical disks have been used as mass storage devices for computer applications, and such optical disks are known as CD-ROMs. The disk which is used as the CD-ROM is modeled after the standard compact disk (CD) that has been developed for audio applications and is basically an audio CD with various improvements and refinements particularly adapted for computer applications. Using such a CD as a standard, the CD-ROM has a data storage capacity of about 600 Mbytes. By using audio CD technology as its basis, the CD-ROM and its disk drive have become relatively inexpensive and are quite popular.
However, since conventional audio CDs with their inherent format and storage capacity have been adapted for CD-ROMs, it has heretofore been difficult to improve the data storage capacity. In typical computer applications, a capacity of 600 Mbytes has been found to be insufficient.
Also, the data transfer rate that can be obtained from audio CDs generally is less than 14 Mbits/sec (Mbps). However, computer applications generally require a transfer rate far in excess of 14 Mbps; but it is difficult to attain a faster transfer rate with conventional CD-ROMs.
Yet another disadvantage associated with conventional CD-ROMs, and which is due to the fact that the audio CD format has been adapted for computer applications, is the relatively long access time associated with accessing a particular location on the disk. Typically, relatively long strings of data are read from audio CDs, whereas computer applications often require accessing an arbitrary location to read a relatively small amount of data therefrom. For example, accessing a particular sector may take too much time for the CD controller to identify which sector is being read by the optical pick-up.
A still further difficulty associated with CD-ROMs, and which also is attributed to the fact that such CD-ROMs are based upon audio CD technology, is the error correcting ability thereof. When audio data is reproduced from an audio CD, errors that cannot be corrected nevertheless can be concealed by using interpolation based upon the high correlation of the audio information that is played back. However, in computer applications, interpolation often cannot be used to conceal errors because of the low correlation of such data. Hence, the data that is recorded on a CD-ROM must be encoded and modulated in a form exhibiting high error correcting ability. Heretofore, data has been recorded on a CD-ROM in a conventional cross interleave Reed-Solomon code (CIRC) plus a so-called block completion error correction code. However, the block completion code generally takes a relatively long amount of time to decode the data, and more importantly, its error correction ability is believed to be insufficient in the event that multiple errors are present in a block. Since two error correction code (ECC) techniques are used for a CD-ROM, whereas only one ECC technique is used for an audio CD (namely, the CIRC technique), a greater amount of non-data information must be recorded on the CD-ROM to effect such error correction, and this non-data information is referred to as xe2x80x9credundantxe2x80x9d data. In an attempt to improve the error correction ability of a CD-ROM, the amount of redundancy that must be recorded is substantially increased.
Therefore, it is an object of the present invention to provide an improved optical disk having particular use as a CD-ROM which overcomes the aforenoted difficulties and disadvantages associated with CD-ROMs which have been used heretofore.
Another object of this invention is to provide an optical disk which exhibits a higher access speed, thereby permitting quick access of arbitrary locations, such as sectors, to be accessed quickly.
A further object of this invention is to provide an improved optical disk having a higher transfer rate than the transfer rate associated with CD-ROMs heretofore used.
A further object of this invention is to improve the storage capacity of an optical disk, thereby making it more advantageous for use as a CD-ROM.
An additional object is to provide an improved optical disk which stores data with reduced redundancy.
Still another object of this invention is to provide an improved recording format for an optical disk which enhances the error correcting ability thereof.
Another object of this invention is to provide an optical disk having a substantially improved recording density, thereby facilitating use of the disk as a CD-ROM.
A further object of this invention is to provide an improved optical disk having data recorded in sectors, with each sector having a sector header that is easily and rapidly read, particularly because the sector header is not encoded in a form which requires a substantially long amount of time before it is successfully decoded and recognized.
Various other objects, advantageous and features of the present invention will become readily apparent from the ensuing detailed description, and the novel features will be particularly pointed out in the appended claims.
In accordance with this invention, an optical disk, a method and apparatus for recording that disk and a method and apparatus for reading data from that disk are provided. The disk has a diameter of less than 140 mm, a thickness of 1.2 mmxc2x10.1 mm, and a plurality of record tracks exhibiting a track pitch in the range between 0.646 xcexcm and 1.05 xcexcm with data recorded in those tracks as embossed pits. The tracks are divided into a lead-in area, a program area and a lead-out are, with table of content (TOC) information being recorded in at least one TOC track in the lead-in area and user information recorded in a plurality of user tracks in the program area. Each track is divided into sectors and the TOC information includes addresses of the start sectors of each user track. The data (both TOC and user information) is encoded in a long distance error correction code having at least eight parity symbols, the encoded data being modulated and recorded on the disk. Preferably, the data is modulated as run length limited (RLL) data.
In the preferred embodiment, the data is recorded with a linear density in the range between 0.237 xcexcm per bit and 0.378 xcexcm per bit. Also, the program area is disposed in a portion of the disk having a radius from 20 mm to 65 mm.
The format of the data advantageously permits rapid access to a desired sector. Reduced redundancy in the recorded data and a higher storage capacity are attained. Advantageously, the optical disk may record data having particular computer application, referred to computer data, or video and audio data, the latter being compressed by the so-called MPEG (Moving Picture Image Coding Experts Group) technique. Audio data which also may be recorded preferably is compressed and then multiplexed with the MPEG-compressed video data.
The error correction code used with the present invention preferably is a long distance code having at least eight parity symbols. ECC techniques which have been used heretofore have relied upon so-called short distance codes in which a block of data is divided into two sub-blocks, each sub-block being associated with a number of parity symbols, such as 4 parity symbols. It is known, however, that 4 parity symbols may be used to correct 4 data symbols, and if 4 data symbols in each sub-block are erroneous, the total number of 8 erroneous data symbols can be corrected. But, if one sub-block contains 5 erroneous data symbols, whereas the other sub-block contains 3 erroneous data symbols, use of the short distance code may be effective to correct only 4 data symbols in the one sub-block, thus permitting a total error correction of 7 data symbols. But, in the long distance code, the block of data is not sub-divided; and as a result, all 8 erroneous data symbols, if present in the long distance coded data, can be corrected.
As another feature of this invention, the RLL code that is used preferably converts 8 bits of input data into 16 bits of data for recording (referred to as 16 channel bits) with no margin bits provided between successive 16-bit symbols. In RLL codes used heretofore, 8 data bits are converted into 14 channel bits and three margin bits are inserted between successive 14-bit symbols. Thus, the present invention achieves a reduction in redundancy.